1. Field of the Invention
The present invention relates to the field of valve actuators and more particularly to gas operated valve actuators.
2. Description of the Prior Art
There are a variety of prior art valve actuators or valve controls which are operated either electrically, hydraulically, or pneumatically. In those valve actuators which permit manual as well as powered operation, there is a direct one-to-one ratio of the hand wheel shaft to the powered shaft which presents a dangerous condition for operating personnel when the hand wheel shaft rotates at the same time that the actuator is in operation. If the hand wheel is not permanently attached to the hand wheel shaft, when power is applied, the hand wheel can fly off injuring both the operators and the equipment. If the hand wheel is permanently mounted to the hand wheel shaft, when power is applied to the actuator, a flywheel effect ensues which will result in an over-driving of the valve damaging the valve or jamming it and making it impossible to open it either manually or by powered operation.
In a gas operated actuator, if some of the power gas is stored in the transmission lines between the actuators cut off valves and limit valves, or if the power gas is not turned off, and the hand wheel is then turned, injury can result since, as soon as the limit valves are open, the gas will be released causing the motor to turn which will then power-drive the band wheel backwards.
In those actuators using spool-type valves, the valves close very slowly as the actuator approaches the limit of its travel. Therefore, at the limit of its travel, torque output of the actuator is reduced; but full torque output up to the moment of shutoff is desirable. Also, in many of the prior art actuators, the power gas is not immediately exhausted to the atmosphere but rather is routed from a motor back through a selector valve. This throttling or choking effect restricts the flow of the gas and reduces the power available. In those actuators that use limit and selector valves having rings mounted on spools, ingredients in natural gas causes the rings to swell resulting in damaged or severed rings and stuck valves. In those actuators in which the limit valve springs are located above the spools, it is a common occurrence for the area around the springs to fill with condensate which thereafter freezes and causes the actuator to fail.
Many actuators are of the cylinder type which usually rely upon a hydraulic manual pump for manual operation. This is undesirable because of possible oil loss, control valve malfunction, and the temperature effect on hydraulic oils. Low pressure cylinder actuators have the added problems associated with pressure regulators and relief valves. Many cylinder actuators on gate valves rely on check valves to prevent unwanted closure and are subject to creepage. A number of cylinder actuators, including rotating vane cylinder actuators, although available in both high and low pressure type, have all the above limitations. Some electrical powered actuators have been modified in an attempt to provide an actuator with the desired features. These actuators use a pneumatic motor coupled to worm gearing which is used in the main drive gearing. The result is a hybrid actuator with unreliable limit control, inefficient gearing and makeshift limit valves. The main disadvantage is the extreme number of the handwheel turns required due to the worm gearing. Gas consumption rates are also high with this type of actuator.